Decembers talk: “Mars, The Prohibition planet” by Dr. J. Wade

Dr
Jon Wade of the OU is an example of the mature student who got hooked on Earth
Sciences – and has that typical enthusiasm of someone who has come late onto
his subject.

Some
intriguing stuff for starters: the nebula history of planet formation
goes back to Immanuel Kant, 1755, but it was Pierre-Simon Laplace who supposed
that the nebula had to be rotating round the baby Sun, developing concentric
rings from which the planets formed. It appears he got this idea from a
rather unusual character, Emanuel Swedenborg, who got the idea from a
dream…(look the rest up.)

So
we think we know about how gravity caused all nebula bits to coalesce then
wham, you have your planetesimals then planets. But how the little bits
of stuff start to aggregate is still not well understood. Once the bits
get bigger then you start to get radioactive decay of Aluminium 26, then the
lumps get bigger and differentiation starts, so you get a denser iron rich core
and silicates on the surface. This happens when the lumps get to about
100km across, he says. Look up Pallasites – odd and lovely meteoritic
lumps that have come from these planetesimals where you get iron and silicates
(usually olivine) mixed up in them.

There
is a lot of rust in this talk: the cores of Mercury, Earth and Mars are
similar but iron in the mantle is hardly seen in Mercury, more in the Earth’s
and even more in Mars’ (weight/atomic percentages being roughly 1%, 8% and 18%
respectively). This means that the planets nearer to the Sun lost their
oxygen more easily.

So,
is core formation the key to planetary habitability? (His words)
How long did the water last? How significant is this rustiness? And
why is Mars so lopsided? Mars’ northern boundary is lower than the
southern, which has lots of craters and a high level of minerals that have
reacted with water. The northern bit is an old ocean basin, the result of
a massive impact. It has a long dead volcano 22km in height. Why no
evidence of tectonic activity? The implication is that there was a lot of
water on Mars between about 4 billion years ago and 3.5 billion years
ago, which means a lot of sedimentary activity took place on Mars in the first
500 million years of its life. Its magnetic field was weak, as were its
tectonics, and there may have been as much as 3km depth of water at one
point. The presence of the iron (more of his lively speculations) means
the water was taken up in the rocks and sediments. Easy to understand why
Mars is the colour it is – yes, it really is the rust – and why the lack of
water prevented life from evolving.

Luckily
the constant upheavals in the Earth’s mantle mean that new stuff keeps coming
to the surface, so it never went the way of Mars. Copper, nickel, iron,
manganese, all stayed stuck deep in Mars’ core. Iron is very important
for life on Earth. However, we did have a boring billion years (his
words) around 2.5 billion years ago, where there was major oxidation (too much
iron again) and a large upsurge in methane in the atmosphere, which killed off
any attempt at incipient life formation. Look at the surface
basalts. He says Earth’s core is gradually growing, but that it’s
crystallisation that’s keeping the heat going, rather than radioactivity.
He says radioactive elements don’t like staying in the core.